This invention relates to step-up and step-down DC--DC conversion circuits using a single IC (integrated circuit). More particularly, this invention relates to step-up and step-down DC--DC conversion circuits using a single IC, without the use of inductors and without the operational problems associated with high inrush currents common to switched capacitor DC--DC converters.
Voltage regulator circuits are used to provide a regulated output DC voltage over a wide range of unregulated input DC voltages and have been implemented using various techniques. In a switching regulator, the flow of power to a load is regulated by controlling the on and off duty-cycle of one or more power switches coupled to the load. There are several existing topologies for performing step-up or step-down DC--DC conversion.
The single-ended inductor based step-up or step-down conversion topology consists of relatively simple circuits where a switch determines whether the voltage applied to an inductor is V.sub.IN or zero. In this manner, the output voltage is a function of the average voltage applied to the inductor. The output voltage varies depending on the configuration of individual components and the class of single-ended inductor circuit. For example, placing the switch in series between the input and the output causes the output voltage to be less than the input voltage. Such a circuit is commonly referred to as a "step-down" or "buck" converter. By placing the switch in parallel with the output, the output voltage can be made to be greater than the input voltage. Such a circuit is commonly referred to as a "step-up" or "boost" converter. Buck and boost circuits may also be combined as a "buck-boost" regulator to provide an inverted-polarity output. One disadvantage of these circuits is that they are inductor based, require numerous external components in addition to IC control, while suffering from EMI, parasitic and stability concerns particular to inductors.
Another known regulator topology is the transformer-coupled switching regulator. This topology provides an increased ability to achieve current or voltage gain, as well as the possibility of isolation between input and output provided by the transformer. As was the case with single-ended inductor regulators, transformer-coupled regulators are also grouped into classes. So-called "flyback converters" use a transformer to transfer energy from input to output. By adjusting the turns ratio (N) between the primary and secondary of the transformer, the regulator may be caused to provide output voltages that are higher or lower than the input voltage. One disadvantage of flyback converters is the high energy that must be stored in the transformer for proper operation. This requires relatively large magnetic cores and may reduce overall circuit efficiency.
The "forward converter" is another class of transformer-coupled regulator. In the forward converter, an additional winding is provided along with other components, such as diodes and capacitors, which essentially eliminates the problem of large stored energy in the transformer core. The additional (or reset) winding normally has a 1:1 turns ratio to the primary winding to help define the switch voltage when the primary switch is turned off. Unfortunately, the turns ratio may limit the duty-cycle of the device to 50% maximum, above which the switch current will typically rise in an uncontrolled manner. If the number of turns on the reset winding is reduced to increase duty-cycle, there is typically a corresponding increase in the level of switch voltage.
It is also possible to provide other combined configurations, such as a "buck-flyback hybrid" converter, in order to attempt to utilize the best properties of different topologies. Previously known buck-flyback converters provide multiple regulated outputs by generating a main output (e.g., 3.3 volts) using a buck converter and adding an auxiliary winding which operates as a flyback converter for a secondary output (e.g., 5 volts). One disadvantage of these converters is the fact that the output power of the auxiliary winding is limited by the output power of the main output. Additionally, when the auxiliary output is heavily loaded, the buck inductor will observe increased output voltage ripple due to the transformer effects of the auxiliary winding. These, as well as other, disadvantages severely limit the input and output operational ranges of these devices. Also, the flyback action of this converter may result in a severely reduced overall efficiency.
Another regulator topology is the switched capacitor switching step-up or step-down converter. Though the switched-capacitor class of step-up or step-down DC--DC converters tend to be the least complex and require the fewest number of external components of all of the topologies, they too are limited in their ability to perform either step-up or step-down function. Moreover, another disadvantage of switched-capacitor converters is that they create large inrush currents from the input voltage, particularly when a large input to output differential voltage exists. Such inrush currents can cause large voltage transients on the input supply, excessive output voltage ripple, and actual over-stress damage to the control IC in extreme cases.
There are few topologies that can perform both step-up and step-down DC--DC conversion. However, these topologies, e.g., SEPIC (single ended primary inductance converter), are inductor or transformer based and, as a result, require numerous external components in addition to the control IC. Consequently, these topologies tend to be more costly and complex than standard step-up or step-down topologies individually. Moreover, these step-up and step-down circuits also suffer from EMI, parasitic and stability concerns particular to inductors.
In view of the foregoing, it would be desirable to provide step-up and step-down DC--DC converter circuits using a common switch network on a single IC that utilizes switched-capacitor techniques.
It would also be desirable to provide step-up and step-down DC--DC converter circuits using a common switch network on a single IC that utilize switched-capacitor techniques while providing inrush current limiting.